The invention at hand relates to the field of power plant technology. It relates to a method for operating a combined cycle power plant having at least two power plant units, each of which power plant units includes a water/steam cycle, as well as to a combined cycle power plant useful for executing the method.
When starting or cold-starting a combined cycle power plant, in which combined cycle power plant the hot waste gases of a gas turbine system are used in order to generate steam within a water/steam cycle in a heat recovery steam generator and work it off via a steam turbine, the heat recovery steam generator, because of its unstable or uncontrollable behavior in the start-up phase up to about 10 (to 20) bar, as well as the thick-walled components of the water/steam cycle (boiler cylinders, steam lines, steam turbines, . . . ) must be considered, with respect to the developing thermal stresses, as critical components that determine the start-up time. The thick-walled containers and components are especially sensitive with respect to large temperature gradients. However, because of the material properties of water or steam, such temperature gradients are created during the start-up or pressure build-up in the system; and this occurs especially in a phase where the system must be left more or less in its natural progression. Only starting at a pressure of about 10 to 20 bar is it practical to intervene by control-technological means. The saturation temperature hereby occurs already at about 180 to 210xc2x0 C. A preheating (during the cold start) or maintaining of the heat (during re-start) is able to keep the stresses at a low level and substantially reduce start-up times in such cases.
U.S. Pat. No. 5,473,898 suggests that for a combined cycle power plant, compressed, heated air from the compressor of the associated gas turbine system is branched off and used to preheat the steam turbine of the water/steam cycle. The disadvantage hereby is that on the one hand the gas turbine system already must have been started up for preheating, and that on the other hand another medium (fluid) is used as a working medium for preheating.
U.S. Pat. No. 5,042,246 suggests an auxiliary steam generator for preheating the steam turbine. However, such a separate auxiliary steam generator requires significant additional equipment expenditure.
In U.S. Pat. No. 5,203,160, steam is removed from an auxiliary steam system in order to preheat the steam turbine of a combined cycle power plant. However, no information is provided regarding the generation of the auxiliary steam.
EP-B1-0 415 300 finally suggests that for a pure steam power plant with several power plant units, a mutual auxiliary steam track is used from which the start-up steam for the individual points at which it is needed can be removed. The start-up steam for the auxiliary steam track is hereby generated in a heat recovery steam generator that is supplied with the hot waste gases of a separate start-up gas turbine. The greatly simplified system schematic of such a system is shown in FIG. 1. The steam power plant 10 comprises several power plant units 12A, . . . , 12D, each of which is connected via valves 14 to a common auxiliary steam track 13. The start-up steam for the auxiliary steam track 13 is generated in a separate start-up steam generator 11 that is connected to the auxiliary steam track 13. If start-up steam is required in one of the power plant units 12A, . . . , 12D, the corresponding valve 14 is opened, and the steam is removed from the auxiliary steam track 13.
It is the objective of the invention to describe a method for operating a combined cycle power plant that comprises several power plant units and in a simple way enables a preheating or maintaining of heat in a power plant unit with little additional expenditure, as well as a combined cycle power plant for executing the method.
The objective is realized according to the present invention. The core of the invention includes connecting the individual power plant units with a continuous-feeder manifold line and removing a heat-transporting fluid, in particular saturated steam, from one or more of the operated power plant units and using it for preheating or maintaining the heat of another power plant unit.
A preferred embodiment of the method according to the invention includes using saturated steam as the heat-transporting fluid since it is especially suitable for the components of the water/steam cycle that are being supplied with steam.
A preferred embodiment of the combined cycle power plant according to the invention is characterized in that saturated steam is used as a heat-transporting fluid, and that the continuous-feeder manifold line is connected by one each saturated steam line with one or more power plant units for saturated steam removal.
In particular, if the power plant units each comprise a gas turbine system with a cooling air cooler, the continuous-feeder manifold line can be connected via a saturated steam line to the cooling air cooler.
However, if the power plant units each comprise a heat recovery steam generator with a steam drum, the continuous-feeder manifold line also can be connected via a saturated steam line to the steam cylinder.
Finally, if the power plant units each comprise a heat recovery steam generator with a separator, the continuous-feeder manifold line can be connected via a saturated steam line to the separator.